Add 1.2.2 release notes

Author: eldryoth

RELEASE_NOTES_1.2.2.md

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+# Fluxheim 1.2.2 Release Notes
+
+## Release Metadata
+
+- Version: `1.2.2`
+- Release date: 2026-05-13
+- Git tag: `v1.2.2`
+- Release type: focused storage-bin cache follow-up
+
+## Summary
+
+Fluxheim `1.2.2` adds the focused slab/bin-style disk cache backend planned
+after `1.2.1`. The existing filesystem disk cache remains the portable default,
+while operators can opt into `cache.disk.backend = "storage-bin"` for large,
+high-churn cache workloads that should avoid one-file-per-object storage.
+
+## Highlights
+
+- Added `cache.disk.backend = "storage-bin"` runtime selection while keeping
+  the existing filesystem backend as the default.
+- Added the storage-bin disk cache backend with manifest files, deterministic
+  bin files, durable object index recovery, reusable free ranges, and LRU
+  eviction parity.
+- Added management parity for storage-bin cache stats, activity reset, exact
+  purge, indexed hard/soft purge, stale purge, cache lookup, and cache warm
+  workflows.
+- Debounced durable storage-bin index writes after insert, eviction, and purge
+  bursts to reduce write amplification under high-cardinality cache fills.
+- Added clean shutdown flushing for pending storage-bin index updates.
+- Added same-key rewrite handling so revalidation or object replacement can
+  release and reuse the previous range.
+- Added conservative tail-bin reclamation so eviction and purge can remove
+  fully-free highest-numbered bin files without moving live objects.
+- Exposed storage-bin pressure stats through admin JSON and Prometheus gauges,
+  including allocated bin bytes, reusable free bytes, free range count, largest
+  free range, and bin file count.
+- Added `examples/cache-storage-bin.toml` plus CI and smoke coverage for the
+  storage-bin backend.
+
+## Validated Scope
+
+- Local gate passed before release prep:
+  - `cargo test -q storage_bin --lib`
+  - focused storage-bin rewrite, drop-flush, and tail-reclaim tests
+  - `cargo check --features profile-observability,acme-client`
+  - `cargo check --no-default-features --features proxy,web,cache,tls-rustls,security`
+  - `cargo clippy --features profile-observability,acme-client --all-targets -- -D warnings`
+  - `sh scripts/smoke_storage_bin_cache.sh`
+  - `cargo run --quiet -- --check-config --config examples/cache-storage-bin.toml`
+  - `perl scripts/check-doc-links.pl`
+  - `scripts/validate-release-metadata.sh`
+  - `git diff --check`
+
+## Known Limits
+
+- The storage-bin backend is opt-in in `1.2.2`; the filesystem backend remains
+  the default until production testing proves the bin format across more
+  deployment shapes.
+- Storage-bin compaction is conservative and only reclaims fully-free tail
+  bins. Moving live objects between bins is reserved for a later compaction
+  pass if production data shows it is needed.
+- Partial streaming admission and range-slice fill remain future cache work.
+- Optional cache encryption at rest, including local-key and OpenBao
+  Transit/KMS key providers, is planned for `1.2.3`.
+- Distributed cache metadata and peer-fill are planned for `1.2.4`.
+- Wasm-based extension points, including cache-rule hooks comparable to VCL/Lua
+  style customization, are planned for `1.4`.
+
+## Checksums And Signatures
+
+Record during the release:
+
+- Commit: `v1.2.2` tag target
+- Local gate: GitHub CI green before tag; local release metadata checks passed
+- CodeQL/code scanning: no open release-blocking alerts before tag
+- Source archive checksums: to be filled
+- Binary checksums: to be filled
+- SBOM checksums: to be filled
+- Reproducible build: to be filled
+- Container digests: to be filled
+- Tag signature: to be filled